The invention relates to a pipe for relining underground pipelines, which is of the composite type with an annularly rigid corrugated outside wall of polymer material, which is attached to the outside surface of a smooth inside wall, which consists of a thermoplastic elastomer so that it can be stretched and compressed.
Pipes of this type are described in the European patent application with publication number 0 213 674.
When an underground pipeline is to be relined by using pipes of this type the pipe is drawn from ground level through a well or manhole into the pipeline, and due to the fact that the inside wall consists of a thermoplastic elastomer the pipe then can be abruptly angled--at an angle of 90.degree. if necessary--when it passes from the well or the manhole into the pipeline, and the pipe can adjust to existing constrictions and direction changes in the underground pipeline more easily than in case of a rigid inside wall. In order to use such relining pipes having a rigid inside wall it is usually necessary to take up wide manholes which allow the pipe to be drawn from ground level into the underground pipeline via the manhole along a rather flat trajectory.
Pipes of the type referred to herein are not without problems, however, in other respects than those relating to the procedure of drawing the pipe into the underground pipeline. In the first place problems arise in the manufacture of the pipe. The pipe is manufactured by coextruding the inside wall and the outside wall, said latter wall at the same time in connection with the extrusion being corrugated by forming and cooling the wall against chains of chilled molds and the smooth inside wall being calibrated and cooled by sliding over a cooling mandrel. The problem involved in the technique of manufacturing the pipe is that the inside wall of a thermoplastic elastomer tends to stick to the cooling mandrel involving the risk of the wall being torn to rags. In the second place there is a problem with respect to the wear resistance of the smooth inside wall of thermoplastic elastomer. This wall must be made substantially thinner than the wall of corresponding smooth, rigid polyethylene or PVC pipes for waste conduits in order that it can be stretched and compressed as is necessary at abrupt direction changes when the pipe is drawn into the underground pipeline, which means that the wear resistance will be a critical factor in this connection. At the same time it can be questioned if a wall of a thermoplastic elastomer can be on a level with a wall of polyethylene or PVC as far as the long term wear resistance is concerned.